1. Field of the Invention
The present application relates to a technique which controls broadcast storms caused by generation of a lot of broadcast frames and multi-cast frames in a communication system.
2. Description of the Related Art
If broadcast frames use up too much bandwidth of a LAN, the network goes down. This situation is a so-called broadcast storm, and it becomes an operational problem of IT systems.
The broadcast storm can be generated, for example, due to false connection of a cable to an L2-Switch (hereinafter, L2-SW) or a failure of an L2-SW. As shown in FIG. 4, when the broadcast storm uses up bandwidth, not only communication between a server and a client or between a server and a server but also control packets between routers are disconnected. For this reason, abnormality of path control in an IT system is caused, which results in multiple failures.
As shown in FIG. 5, there is a conventional technique which represses broadcast storms. A receiving filter (a filter which discards received data) is set for each interface of each L2-SW. The number of packets or bytes of the broadcast frames (hereinafter, BC frame) and the multi-cast frames (hereinafter, MC frame) is counted, and a total value per time slot is calculated. The time slot is a unit of predefined period (for example, one sec). When the total value exceeds a predetermined threshold, a receiving filter (a filter which discards received data) for the BC frames and the MC frames is set in an interface where excess over a threshold occurs.
A technique relating to a bridge apparatus eliminates a double path caused by a loop, so as to repress an unstable state of a network and be capable of preventing deterioration of transmission service of the network.
An operation of a switch (hereinafter, SW) having a storm control function in a conventional interface is described according to a time slot with reference to FIG. 5. The storm control function monitors the number of received frames in the BC frames or the MC frames, and detects abnormal traffic when the number of received frames exceeds a prescribed threshold.
At time slot 1, the number of received frames in the BC frames and the MC frames does not exceed the threshold, and thus normal operation is performed or only a frame of a control system is communicated on a network.
At time slot 2, when the SW detects that the number of the BC frames and the MC frames exceeds the threshold, the SW sets the receiving filter (which discards the BC frames and the MC frames) to ON at the next time slot for the interface where the excess over the threshold is detected.
At time slot 3, when the received BC frames and MC frame do not exceed the threshold, the SW sets the receiving filter to OFF (cancel the receiving filter) at the next time slot. Even when the receiving filter is set, the SW continues counting the flow rate of communication.
At time slot 4, since the received BC frames and MC frames do not exceed the threshold, the receiving filter of the SW is kept OFF at the next time slot.
At time slot 5, when the SW detects that the frames exceed the threshold in any interface, the SW sets the receiving filter to ON similarly to time slot 2.
The receiving filter which is set by the storm control function is a filter for a destination MAC address, and its filter conditions are as follows:                a frame where MAC is a broadcast address (FF-FF-FF-FF-FF-FF) is to be discarded;        a frame where MAC is MC address “01-00-5E-00-00-00” to “01-00-5E-7F-FF-FF” is to be discarded (this range includes an address for a path post frame of OSPF (one of an IP path control method); and        an address of a control system multi-cast frame of a layer 2 such as BPDU is not to be discarded.        
The storm control function, however, produces a problem when a filter is set for excessive interfaces.
FIG. 6 illustrates an operation of the storm control function provided to the conventional SW in the whole network, and this problem is described. The storm control function is ON and the filter function is effective in all the interfaces of SWs.
At time slot 1, a storm is caused by generation of many BC frames (or MC frames) generated from the SW-1 due to failure of the first SW (hereinafter, the n-th SW is described as SW-n). Each SW detects that the BC frames or the MC frames exceed the threshold in an interface (SW-1-side interface) which receives the storm.
At time slot 2, the receiving filter is set to ON for the BC frames in the interface where excess of the BC frames over the threshold is detected. At this time slot, the SW-1 is still transmitting the storm, but the BC frames are discarded by the receiving filter of the SW-2. For this reason, the SW-3 to SW-n do not detect the storm. That is to say, the excess over the threshold does not occur in the SW-3 to SW-n.
At time slot 3, the ON state of the receiving filter in the SW-1-side interface of the SW-2 is continued, and the receiving filters in the SW-1-side interfaces of the SW-3 to SW-n are cancelled (the receiving filters are set to OFF).
At time slot 4, the storm generated from the SW-1 is stopped. Therefore, the BC frames do not exceed the threshold at SW-2.
At time slot 5, the SW-2 cancels the setting of the receiving filter in the SW-1-side interface (the SW-2 sets the receiving filter to OFF).
Thereafter, when a storm is generated from the SW-1 intermittently due to a failure of the SW-1, for example (it is highly probable that the storm is generated or stopped from the SW-1 at a periodic interval due to the failure of the SW-1 and the like), a current state is returned to the state of time slot 1.
A problem in the example of FIG. 6 is time slot 2 after the state is returned to the state of time slot 1. That is to say, the storms which are generated from the SW-1 intermittently are discarded by setting the receiving filter to ON at the SW-2. For this reason, after the BC frames are discarded at SW-2, the receiving filters at the SW-3 to SW-n are not substantially necessary. Further, since the BC frames and the MC frames necessary for the communication such as ARP (Address Resolution Protocol) and path control frames are discarded by the SW-3 to the SW-n, this causes multiple failures.
This is because the storm control function does not take a positional relationship between the respective SW and a generating source into consideration, and sets the receiving filter based on only the flow rate of storms as information for making a decision.
In order to solve the above problems, it is an object to provide a technique which controls a communication system so that a receiving filter for storms from one generating source is set only in a SW closest to the generating source, and receiving filters are not set in the other SWs.